Ocean Island arcs Ocean-ocean collision zones
Volcanic Rocks of Island Arcs • Complex tectonic situation and broad spectrum • High proportion of basaltic andesite and andesite • Most andesites occur in subduction zone settings
Figure 16-6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.
Tholeiitic vs. Calc-alkaline differentiation C-A shows continually increasing SiO2 and lacks dramatic Fe enrichment Tholeiitic magmas: shallow partial melting of mantle Calc-alkaline: restricted to subduction zones. Why?
Major Elements and Magma Series • Tholeiitic (MORB, OIB) • Alkaline (OIB) • Calc-Alkaline (~ restricted to subduction zone)
Island Arc Petrogenesis Figure 16-11b. A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.
Island Arc Petrogenesis • Altered ocean crust dehydrates at 50 km • Chlorite dehydrates first • Amphibole dehydrates at 110 km (point A) • Slab metamorphosed until 80-100 km • Water rises into mantle wedge • Hydrous mantle heats, melts, rises
How to get calc-alkaline trend • Crystallize hornblende, anorthite (Ca-rich plag), olivine • How to get hornblende to crystallize? • Pond magmas in the crust • Get farther down Bowen’s reaction series olivine Calcic plagioclase (Spinel) Mg pyroxene Calcic-alkalic plagioclase Continuous Series Mg-Ca pyroxene alkali-calcic plagioclase Discontinuous Series amphibole alkalic plagioclase biotite Temperature potash feldspar muscovite quartz
Magma trends in island arcs • 1st magma: tholeiites • More primitive • Melts of mantle like OIB’s • 2nd: calc-alkaline • Crust gets thicker • Magmas stall out more
Chapter 16. Island Arc Magmatism Activity along arcuate volcanic island chains along subduction zones Distinctly different from the mainly basaltic provinces thus far Composition more diverse and silicic Andesite most common rock Basalt generally occurs in subordinate quantities Also more explosive than the quiescent basalts Strato-volcanoes are the most common volcanic landform
Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another • The initial petrologic model: • Oceanic crust is partially melted • Melts rise through the overriding plate to form volcanoes just behind the leading plate edge • Unlimited supply of oceanic crust to melt
Ocean-ocean Island Arc (IA) Ocean-continent Continental Arc or Active Continental Margin (ACM) Figure 16-1. Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate. PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.
Structure of an Island Arc Figure 16-2. Schematic cross section through a typical island arc after Gill (1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6joules/cm2/sec)
Structure of an Island Arc • Subduction rate 0.9-10.8 cm/yr • Subduction angle 30-90° (45 average) • Younger the subduction slab, the shallower dip • Earthquakes as deep at 700 km • Volcanoes 110 km above slab
Major Elements and Magma Series a. Alkali vs. silica (alkaline rocks minor) b. AFM (both types) c. FeO*/MgO vs. silica diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370.
Sub-series of Calc-Alkaline • K2O is an important discriminator 3 sub-series • Low K: usually basalt • Medium and high-K are andesites Figure 16-4. The three andesite series of Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington).
Figure 16-6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K, diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.
Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.
Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
May choose 3 most common: • Low-K tholeiitic 6 sub-series if combine tholeiite and C-A (some are rare) • Med-K C-A • Hi-K mixed Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-alkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981).
Magmas are differentiated (not primary) • Only most Mg-rich are nearly primitive mamgas • Spread in data is due to fractionation of Ol, opx, cpx, plag • Similar to MORB magma source it seems
Magma source • Need little Fe-enrichment, Na and K enrichment • If melting a depleted magma source, need water to facilitate melting • More water: can have amphiboles, biotite • More water: changes phase diagram • Larger olivine field (less Mg, Fe, Ni, Cr in melt) • Smaller plag field (more Na and K in melt)
Calc-alkaline differentiation • Early crystallization of an Fe-Ti oxide phase Probably related to the high water content of calc-alkaline magmas in arcs, dissolves • The crystallization of anorthitic plagioclase and low-silica, high-Fe hornblende is an alternative mechanism for the observed calc-alkaline differentiation trend
Petrogenesis of Island Arc Magmas • Why is subduction zone magmatism a paradox?
Rocks heat as subducted • On normal geotherm, basalt melts at 40 km • In subduction zones, not until 200 km • All volcanoes sit 110 km above slab
Of the many variables that can affect the isotherms in subduction zone systems, the main ones are: 1) the rate of subduction 2) the age of the subduction zone 3) the age of the subducting slab
Typical thermal model for a subduction zone • Isotherms will be higher (i.e. the system will be hotter) if a)the convergence rate is slower b) the subducted slab is young and near the ridge (warmer) c) the arc is young (<50-100 Ma according to Peacock, 1991) yellow curves = mantle flow Figure 16-15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).
The principal source components island arc magmas 1. The crustal portion of the subducted slab 1a Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies) 1b Subducted oceanic and forearc sediments 1c Seawater trapped in pore spaces Figure 16-15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).
The principal source components Island arc magmas 2. The mantle wedge between the slab and the arc crust 3. The arc crust Figure 16-15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, 8309-8319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).
Left with the subducted crust and mantle wedge • The trace element and isotopic data suggest that both contribute to arc magmatism. How, and to what extent? • Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in the thermal regime shown • water plays a significant role in arc magmatism
A multi-stage, multi-source process • Dehydration of the slab provides the LIL enrichments • These components, plus other dissolved silicate materials, are transferred to the wedge in a fluid phase (or melt?) • The mantle wedge provides the depleted and compatible element characteristics
Trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source • The flat HREE pattern argues against a garnet-bearing (eclogite) source • Thus modern opinion has swung toward the non-melted slab for most cases
10Be created by cosmic rays + oxygen and nitrogen in upper atmos. • Earth by precipitation & readily clay-rich oceanic seds • Half-life of only 1.5 Ma (long enough to be subducted, but quickly lost to mantle systems). After about 10 Ma 10Be is no longer detectable • 10Be/9Be averages about 5000 x 10-11 in the uppermost oceanic sediments • In mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14
B is a stable element • Very brief residence time deep in subduction zones • B in recent sediments is high (50-150 ppm), but has a greater affinity for altered oceanic crust (10-300 ppm) • In MORB and OIB it rarely exceeds 2-3 ppm
10Be/Betotal vs. B/Betotal diagram (Betotal9Be since 10Be is so rare) Figure 16-14.10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123.